Ionic Liquid Functionalized Reduced Graphite Oxide / TiO2 Nanocomposite for Conversion of CO2 to CH4

ABSTRACT

An ionic liquid functionalized reduced graphite oxide (IL-RGO)/TiO 2  nanocomposite was synthesized and used to reduce CO 2  to a hydrocarbon in the presence of H 2 O vapor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. provisional patent application No. 61/724,578 entitled “Ionic Liquid Functionalized Reduced Graphite Oxide/TiO₂ Nanocomposite for Conversion of CO₂ to CH₄” filed on Nov. 9, 2012, the contents of which are incorporated by reference as if set forth in its entirety herein for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The invention was made with government support under 1253443 awarded by the National Science Foundation. The United States government has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to a photocatalytic nanocomposite, a related method of making the photocatalytic nanocomposite, and a method of using the photocatalytic nanocomposite to covert carbon dioxide into methane.

BACKGROUND OF THE INVENTION

Photocatalytic materials are drawing significant attention because of their potential for solving environmental and energy problems. Among these problems are finding ways to address the contributions of CO₂ to global warming while still permitting increased energy consumption, as energy plays a critical role in the quality of life improvement and economic prosperity.

One potential avenue for reduction of greenhouse gases such as CO₂ has been CO₂ photoreduction using a photocatalyst in which CO₂ is reduced to various less harmful products over the photocatalyst that is activated by UV radiation. To date, titanium dioxide (TiO₂) is one of the most studied photocatalysts because it has shown the most efficient photocatalytic activity, highest stability, low cost, as well as low toxicity. In the photocatalytic reaction, electrons and holes are produced from TiO₂ under UV irradiation. The electrons and holes subsequently interact with reactants (including CO₂) to form the products.

However, CO₂ photoreduction has only been performed using titanium dioxide (TiO₂) with limited or qualified success. One of the problems in using unmodified TiO₂ as a photocatalyst is that electron and hole recombination leads to low photoconversion efficiency. Although various modifications to the catalytic structure have been attempted, none of the modifications have created a commercially and industrially viable structure for CO₂ photoreduction.

Hence, a continued need exists for a photocatalyst that improves the kinetics of a photoreduction reaction of CO₂.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a photocatalytic nanocomposite is provided that includes a reduced graphite oxide, a photocatalytic metal oxide in nanoparticle form, wherein the photocatalytic metal oxide in nanoparticle form is dispersed in the reduced graphite oxide. The photocatalytic metal oxide may be TiO₂ and may be in a mixed phase form including rutile and anatase. The reduced graphite oxide may be a powder.

The photocatalytic nanocomposite may further include an ionic moiety attached to the reduced graphite oxide. The ionic moiety may comprise R₁R₂R₃ in which R₁ is NH, R₂ is alkylene, and R₃ is a cationic group. In some forms, R₂ may be C₁ to C₅ alkylene, and R₃ may be an imidazole ring protonated or substituted at a nitrogen atom. In one more specific form, R₂ may be propylene and R₃ may be an alkyl-substituted imidazole ring.

In some instances, the reduced graphite oxide may be an ionic liquid functionalized reduced graphite oxide formed by attaching a NH₂-terminated ionic liquid to the reduced graphite oxide.

According to another aspect of the invention, a method of making a photocatalytic nanocomposite of this type is provided. The method of making a photocatalytic nanocomposite includes oxidizing graphite to form a reduced graphite oxide and mixing the reduced graphite oxide with TiO₂ nanoparticles to form the photocatalytic nanocomposite. The reduced graphite oxide may be functionalized with a NH₂-terminated ionic liquid to form an ionic liquid functionalized reduced graphite oxide before the step of mixing. The NH₂-terminated ionic liquid may an imidazole such as, for example, a 1-butyl-3-methylimidazolium-based ionic liquid (e.g., 1-butyl-3-methylimidazolium chloride).

According to still another aspect of this invention a method of CO₂ photoreduction is provided. The method of CO₂ photoreduction includes contacting reactants of CO₂ and H₂O over a photocatalytic nanocomposite of the type described herein and reacting the CO₂ and H₂O over the photocatalytic nanocomposite to produce products including CH₄. In some forms, the products may be substantially free of CO gas. In some forms, the CH₄ may have a production rate in excess of 10 μmol/g catalyst-hr or in excess of 250 μmol/g catalyst-hr.

According to one specific example, an ionic liquid functionalized reduced graphite oxide (IL-RGO)/TiO₂ nanocomposite was synthesized and used to reduce CO₂ to a hydrocarbon in the presence of H₂O vapor. IL-RGO was synthesized through chemically attaching the ionic liquid, 1-butyl-3-methylimidazolium chloride (C₄mimCl), to a graphite oxide (GO) surface and simultaneously reducing GO to RGO in a basic reaction environment. As a comparison, reduced graphite oxide (RGO) was synthesized using the same procedure, but without adding the ionic liquid. The SEM image revealed that IL-RGO/TiO₂ contained a homogeneous dispersion of graphite flakes with TiO₂ nanoparticles. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was used to study the conversion of CO₂ and H₂O vapor over the IL-RGO/TiO₂ catalyst. Under UV-Vis irradiation, two new peaks were detected in the infrared spectrum after just 40 seconds of irradiation. The intensities of these peaks continuously increased in subsequent spectra that were taken under longer irradiation time. The two peaks detected as products were characteristic of CH₄. Background experiments that were conducted confirmed that CH₄ was indeed generated from the reduction of CO₂ over the IL-RGO/TiO₂ catalyst surface. A CH₄ production rate of 279 μmol/g catalyst-hr (as measured using DRIFTS) over a 55 minute period was calculated. These findings suggest the direct, selective formation of CH₄ in the absence of CO.

These and still other advantages of the invention will be apparent from the detailed description and drawings. What follows is merely a description of some preferred embodiments of the present invention. To assess the full scope of the invention the claims should be looked to as these preferred embodiments are not intended to be the only embodiments within the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the overall synthesis of ionic liquid functionalized reduced graphite oxide (IL-RGO).

FIG. 2 shows the Raman spectra of (a) GO and (b) IL-RGO.

FIG. 3 shows XPS spectra for (a) wide scan survey of IL-RGO/TiO₂, (b) high resolution XPS spectrum of N is, (c) high resolution XPS spectrum of C1s in GO, and (d) high resolution XPS spectrum of C1s in IL-RGO.

FIG. 4 shows X-ray Diffraction (XRD) peaks of (a) GO, (b) RGO, and (c) IL-RGO, and (d) XRD peaks of IL-RGO/TiO₂ scanning from 2θ=20°˜70°.

FIG. 5 shows SEM images of (a) IL-RGO/TiO₂ and (b) RGO/TiO₂.

FIG. 6 shows the IR spectra of IL-RGO/TiO₂ surface with CO₂ and H₂O vapor before and after UV-Visible light irradiation. The IR spectrum of IL-RGOITiO₂ was used as a background. The IR spectra were offset for clarification.

FIG. 7 shows the IR spectra of IL-RGO/TiO₂ with H₂O but without CO₂ before and after 30 minute UV-Visible irradiation.

FIG. 8 shows energy level diagrams for single phase and mixed phase TiO₂.

FIG. 9 shows and energy level diagram for the one embodiment of the IL-RGO/TiO₂ structure.

FIG. 10 shows the concentration of methane over IL-RGO/TiO₂ and regenerated sample by cleaning the surface using N₂ at different UV-Visible irradiation times.

DETAILED DESCRIPTION OF THE INVENTION

Carbonaceous nanomaterials have unique properties and the potential to control the structural and electrical properties of photocatalysts. The presence of a carbon material such as carbon nanotubes (CNTs) or graphene might potentially reduce electron and hole recombination in the photocatalyst via transport of the electrons to the conductive carbon material. By improving the separation of the charges, recombination may be avoided thereby enhancing the photoconversion efficiency of TiO₂. Indeed, to date some nanocarbon (for example, CNTs or graphene)/TiO₂ composites have shown improved photocatalytic activity over TiO₂ in various applications (for example, the photooxidation of environmental pollutants).

As compared to cylindrical CNTs, planar graphene may have a smaller electron transfer barrier. As a result, the electron-hole recombination may be less. Graphene is an atomic sheet of sp²-bonded carbon atoms that are arranged into a honeycomb structure. The high surface area of graphene may increase the adsorption of reactants and provide more active sites. Nearly 90% enhancement in photocurrent was seen for reduced graphene oxide described below, which serves as an electron collector and transporter in the graphene-TiO₂ composite. A significant enhancement in the reaction rate for the degradation of methylene blue was observed using a P25 (Degussa)-graphene composite material in contrast with a bare P25 (Degussa) or a P25 (Degussa)-CNT material with the same carbon content. In addition, a decrease in charge transfer resistance of graphene/P25 (Degussa) sample was observed versus P25 (Degussa) alone.

Presently, there are only a few examples regarding the use of TiO₂ nanocarbon materials for the conversion of carbon dioxide (CO₂) and H₂O vapor to fuels. Xia et al. synthesized multi-walled carbon nanotube (MWCNT) supported TIO₂ and investigated its photocatalytic activity in the reduction of CO₂ with H₂O (Xia, X. H.; Jia, Z. H.; Yu, Y.; Liang, Y.; Wang, Z.; Ma, L. L., Preparation of multi-walled carbon nanotube supported TiO₂ and its photocatalytic activity in the reduction of CO₂ with H₂O. Carbon 2007, 45, 717-721.). Sol-gel and hydrothermal methods were used to synthesize the MWCNT/TiO₂ composite. Both syntheses methods led to the formation of C₂H₅OH, HCOOH and CH₄. The total carbonaceous yields of the products were higher than the reported yields in the literature, which focused on using other materials to modify TiO₂ rather than carbon (i.e., Cu doping, Cu—Fe co-doping TiO₂/porous silica, TiO₂ nanotube). This result suggests that carbon-containing TiO₂ materials may be better than other materials in terms of enhancing CO₂ photoconversion efficiency.

It appears there has been only one instance in which an externally mixed graphene/TiO₂ material was applied to CO₂ photoreduction. Liang, Y. T.; Vijayan, B. K.; Gray, K. A.; Hersam, M. C., Minimizing Graphene Defects Enhances Titania Nanocomposite-Based Photocatalytic Reduction of CO₂ for Improved Solar Fuel Production. Nano Letters 2011, 11, 2865-2870. A solvent exfoliated graphene/P25 thin film and a thermally reduced graphite oxide/P25 thin film were used as the photocatalysts, and CH₄ was the major product identified. The CH₄ production rates of exfoliated graphene/P25 thin film and a thermally reduced graphite oxide/P25 thin film were 8.1 μmol/m²-hr and 1.8 μmol/m²-hr, respectively. The results indicate that TiO₂/graphene may be selective towards CH₄ production but provided low reaction rates.

In this disclosure, a novel synthesis method was used to create an improved graphene type carbon/TiO₂ composite material for CO₂ photoreduction, and the product of the CO₂ photocatalytic reaction in the presence of water vapor was studied. Instead of adding TiO₂ and the graphene type material separately, a nanocomposite material was formed. Graphene is a single atomic layer of sp² carbon structure. In the disclosed method and resulting structure, carbon layers were separated by oxidizing natural graphite to form graphite oxide and subsequently functionalizing graphite oxide with a NH₂-terminated ionic liquid. At the same time graphite oxide was reacted to form reduced graphite oxide (RGO) in a basic reaction environment. The high solubility of the ionic liquid in water makes it possible for an ionic liquid functionalized reduced graphite oxide (IL-RGO) to mix well with TiO₂ nanoparticles in water. Moreover, the ionic liquid, which is functionalized to the RGO surface, may introduce surface charge to the RGO. The charge repulsion may help to further separate the graphite layers. The high surface area of the layer-separated graphite material may enhance the adsorption of the reactants, i.e. CO₂ and H₂O vapor, thus creating more reactive sites.

In addition, NH₂-ionic liquid cations may significantly enhance the ability to attract CO₂ via amine group-CO₂ interactions. Therefore, the amine-functionalized ionic liquid may also enhance the adsorption of CO₂. Previous experimental results indicate that the C₄mimCl ionic liquid does not interact with alkanes. CH₄ is a potential product of CO₂ photoreduction using TiO₂ or modified TiO₂ catalyst. Therefore, the ionic liquid may be able to selectively adsorb the reactant, CO₂, but quickly dissociate the potential product, CH₄, thus promoting the photoreduction of CO₂.

The IL-RGO/TiO₂ nanocomposite was applied to reduce CO₂ in the presence of H₂O vapor. The products of CO₂ photoreduction over the IL-RGO/TiO₂ was compared with the product formation data that were reported in the literature using pristine TiO₂ (P25) and other modified TiO₂ in order to provide an initial examination of the selectivity towards different products.

In the examples provided below, the new method was used to synthesize a carbon/semiconductor composite material via attaching ionic liquid to graphite oxide surface to obtain the ionic liquid functionalized reduced graphite oxide (IL-RGO), and mixing it with TiO₂ nanoparticles in solution. The successful synthesis of this material was confirmed by Raman spectra, XRD, XPS and SEM. Comparing RGO without functionalized ionic liquid, the IL-RGO layers were separated and IL-RGO flakes can be clearly seen in the SEM images. In addition, the SEM images showed that TiO₂ nanoparticles are dispersed with IL-RGO flakes. The photoreduction of CO₂ over IL-RGO/TiO₂ in the presence of H₂O vapor was investigated using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). CH₄ was formed after just 40 seconds of UV-Vis irradiation over the catalyst of IL-RGO/TiO₂. The IR features of CH₄ increased as the irradiation time increased. However, no product was found for the photoreduction of commercial P25 under the same experimental conditions. Therefore, the presence of IL-RGO significantly enhances the photocatalytic activity of P25 due to the enhanced electron-hole separation. In addition, CH₄ was found to be the only product for IL-RGO/TiO₂.

The regeneration and reuse of IL-RGO/TiO₂ catalyst for CO₂ photoreduction was also investigated. The regenerated catalyst can produce almost identical amount of CH₄ at the same irradiation times. A CH₄ production rate of 279 μmol/g catalyst-hr over a 55 minute period was calculated.

The following examples are presented for illustrative purposes only, and are not intended to limit the scope of the present invention in any way.

EXAMPLES Synthesis of the 1 L-RGO/TiO₂ Composite

Synthesis of the ionic liquid reduced graphite oxide/TiO₂ (IL-RGO/TiO₂) composite involved three basic steps: synthesis of graphite oxide, functionalization and reduction of the graphite oxide in order to make the material more conductive, and addition of the TiO₂ to allow for photoactivity.

Synthesis of Graphite Oxide

Graphite Oxide (GO) was synthesized from natural graphite powder (325 mesh, Alfa Aesar) by the method of Hummers and Offeman (Hummers, W. S.; Offeman, R. E., Preparation of Graphitic Oxide. Journal of the American Chemical Society 1958, 80, 1339-1339). Prior to synthesis using Hummers and Offeman's method, the graphite powder was pre-oxidized. Otherwise, incompletely oxidized graphene-core/GO-shell particles were observed in the final product. The pre-oxidation procedure followed the method of Kovtyukhova et al. (Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D., Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations. Chemistry of Materials 1999, 11, 771-778.) Briefly, the pre-oxidation process used concentrated H₂SO₄, K₂S₂O₈ and P₂O₅ to oxidize the graphite powder. The resultant product was subsequently thermally isolated and allowed to cool to room temperature. After cooling, the product was diluted and washed with distilled water until the water's pH became neutral. The product was dried in air at ambient temperature overnight and subjected to oxidation by Hummers and Offeman's method. The pre-oxidized graphite powder was further oxidized using 0° C. concentrated H₂SO₄ and KMnO₄. The reaction was terminated by the addition of a large amount of distilled water and 30% H₂O₂ solution. The mixture was centrifuged with 1:10 HCl solution to remove metal ions. Also, additional distilled water washing was done to the mixture until a neutral pH was achieved. The mixture was dark brown in color. The GO was added to distilled water and sonicated for 15 minutes to separate the GO layers. Finally, the GO sample was obtained by centrifugation of the GO solution at 5000 rpm for 30 minutes.

It is observed that this oxidation step may help to prevent the graphite from aggregating during mixing, as might occur when using natural graphite flakes. This is evidenced by SEM images in the results below in which the flakes remain separated.

Ionic Liquid Functionalized Reduced Graphite Oxide (IL-RGO)

The overall reaction for the synthesis of IL-RGO is presented in FIG. 1. A NH₂-terminated ionic liquid of 1-butyl-3-methylimidazolium chloride (NH₂—C₄mimCl) was synthesized using a process that has been reported previously for the synthesis of NH₂-terminated 1-butyl-3-methylimidazolium bromide (NH₂—C₄mimBr). See e.g.: Yang, H. F.; Shan, C. S.; Li, F. H.; Han, D. X.; Zhang, Q. X.; Niu, L., Covalent functionalization of polydisperse chemically-converted graphene sheets with amine-terminated ionic liquid. Chemical Communications 2009, 3880-3882; and Zhang, Y. J.; Shen, Y. F.; Yuan, J. H.; Han, D. X.; Wang, Z. J.; Zhang, Q. X.; Niu, L., Design and synthesis of multifunctional materials based on an ionic-liquid backbone. Angewandte Chemie-International Edition 2006, 45, 5867-5870.

First, 3-chloropropylamine hydrochloride (Sigma Aldrich, 98%) and 1-methylimidazole (Sigma Aldrich, 99%) were added to ethanol (Sigma Aldrich, ≧99.5%). The mixture was then refluxed under nitrogen for 24 hours. The resulting turbid mixture was purified by re-crystallization from ethanol and ethyl acetate as anti-solvent. Finally, the resulting ionic liquid was dried under N₂ at 60° C. overnight.

The C₄mimCl functionalized RGO synthesis is based on an epoxide ring-opening reaction between GO and NH₂—C₄mimCl. NH₂—C₄mimCl was added into a GO dispersed solution. The salt effect of the GO sheet occurred due to the presence of the ionic liquid. The epoxide ring-opening reaction can be catalyzed by a base. Therefore, KOH was added into the turbid mixture solution. The solution was subjected to ultrasonication for 30 minutes. Lastly, the homogeneous solution was vigorously stirred at 80° C. for 24 hours. The resulting solution was washed using ethanol and distilled water several times until the pH was neutral. The resulting solution was subjected to the IL-RGO/TiO₂ nanocomposite material synthesis process. The IL-RGO solution was dried at room temperature for IL-RGO material characterization. For comparison, RGO was synthesized using the same procedure, but without adding the NH₂—C₄mimC;.

IL-RGO/TiO₂ Nanocomposite Synthesis

TiO₂ nanoparticles (Evonik P25) were mixed with distilled water and 1-Butyl-3-methylimidazolium tetrafluoroborate (C₄mimBF₄, Sigma Aldrich, 98%) (H₂O:C₄mimBF₄=9:1 by volume) to make a TiO₂ suspension. Before making the IL-RGO/TiO₂ composite, the IL-RGO solution was ultrasonicated for 30 minutes and stirred for 1 hour. Later, the IL-RGO solution was added to the TiO₂ suspension and vigorously stirred for one hour. The mass percentage of IL-RGO was 3.5% of the composite material. The IL-RGO/TiO₂ mixture was washed until the pH became neutral. Then the mixture was ultrsonicated for 30 minutes and dried at 80° C. overnight. Eventually, the sample was ground to powder for use in the CO₂ conversion experiments. For comparison, the RGO/TiO₂ was synthesized using the same procedure.

Material Characterization

Raman spectra of GO and IL-RGO were collected using a custom built Raman spectrometer in a 180° geometry. The sample was excited using a 100 mW compass 532 nm laser. The data were collected using an Acton 300i spectrograph and a back thinned Princeton Instruments liquid nitrogen cooled CCD detector. The spectral resolution was 3.5 cm⁻¹. X-ray Diffraction (XRD) data were collected with a high resolution X-ray diffractometer (PANALYTICAL X′PERT PRO) using Cu-Kα radiations and an X′celerator detector. Scanning electron microscopy (SEM) was performed using an XL30 ESEM-FEG. X-ray photoelectron spectroscopy (XPS) was performed using a VG ESCALAB 220i-XL aluminum-Kα (1486.6 eV) X-ray source.

The Raman spectra of GO and IL-RGO are shown in FIG. 2. In the Raman spectrum of GO in FIG. 2 a, the G band at 1580 cm⁻¹ is related to the in-plane vibration of the sp² bonded carbon atoms. The D band at 1339 cm⁻¹ is associated with the vibration of sp³ bonded carbon atoms, which corresponds to the disordered structure of the GO. The D/G band intensity ratio of GO is 1.03. In the Raman spectrum of IL-RGO in FIG. 2 b, the D/G band intensity ratio is 0.94. A decrease of the D/G band intensity ratio of IL-RGO suggests that part of the disorder structures were restored to in-plane sp² structures. The restoration of sp² carbon structure indicates an increase in the conductivity of the material. Thus, IL-RGO is likely to transfer the electrons produced from TiO₂ under UV irradiation.

X-ray spectroscopy (XPS) was used to characterize the chemical composition of the material. The XPS spectra are presented in FIG. 3. The wide scan survey in FIG. 3 a shows that all the expected elements, Ti, O, C and N, are present in the IL-RGO/TIO₂ sample. The high resolution XPS spectra of the IL-RGO/TiO₂ sample were examined for the presence of the anion of the functionalized ionic liquid, Cl⁻, or the elements from the solvents used in the synthesis process. No peak associated with Cl⁻ or any element from the solvents used in the synthesis was found. Therefore, Cl⁻ and solvents used were completely washed out of the sample. The high resolution XPS spectrum of N1s in FIG. 3 b shows that the N1s band appears at 401.7 eV, with a lower binding energy shoulder at 399.8 eV. This XPS feature of N1s is shown in both the IL-RGO/TiO₂ sample and the IL-RGO without adding TiO₂. This confirms the presence of the IL-NH₂ unit in IL-RGO. In addition, the small peak of C—N at 286.3 eV in the high resolution XPS spectrum of C1s in the IL-RGO sample in FIG. 3 d further confirms that the NH₂ terminated ionic liquid was present in the sample.

The atomic concentration ratio of carbon to oxygen (C/O) determined using the XPS data of GO was 0.8, while the C/O ratio for IL-RGO determined using the XPS data was 1.4. The atomic concentration of the C—N peak shown in FIG. 3 d contributes to 2.6% of the total carbon from IL-RGO. According to the molecular structure of C₄mimCl, the maximum percentage of carbon content that could be introduced to IL-RGO by simply attaching the ionic liquid is 6.0%. If it is assumed that the atomic concentration of oxygen does not change in the process of GO conversion to IL-RGO, the maximum C/O ratio should be 0.9. However, the actual C/O ratio of IL-RGO determined using XPS data is 1.4, suggesting that a large number of oxygen groups disappeared in the synthesis of the IL-RGO sample. Hence, GO was significantly reduced.

High resolution XPS spectra of C1s in GO and IL-RGO are shown in FIGS. 3 c and 3 d, respectively. Carbon has multiple binding configurations, including graphite C═C, C═O, C—OH, and C in the epoxide/ether. Comparing the peaks for different binding configurations, the atomic concentration of the graphite peak (C═C) in GO is 18.6% of carbon in all binding configurations. The maximum atomic concentration of the graphite peak contributed from the attached ionic liquid is 2.6%. However, the atomic concentration of graphite peak in IL-RGO accounts for 50.4% of that of the carbon in all binding configurations. This confirms that partial sp² graphite structures were restored. In addition, when the IL-RGO was synthesized from GO, the color of the sample changed from dark-brown to dark-gray. The change in color also strongly suggests that GO was reduced.

Turning now to FIG. 4, the X-ray diffraction (XRD) data show that the diffraction peak of GO appears at 28=12.2° in FIG. 4 a. This corresponds to an average interlayer space of 0.72 nm. The XRD peak for RGO without functionalized IL appears at 2θ=12.7° in FIG. 4 b, corresponding to the average interlayer spacing of 0.70 nm, whereas IL-RGO has a weak and broad diffraction peak at 2θ=11° in FIG. 4 c. As compared to GO, the slightly reduced interlayer space of RGO is likely due to the decreased number of oxygen groups. The calculated interlayer spaces of GO and RGO demonstrate that the interlayer spaces are similar within the GO structure and RGO structure. Different from the sharp XRD peaks of GO in FIG. 4 a and RGO in FIG. 4 b, the broad X-ray diffraction peak of the IL-RGO sample in FIG. 4 c and its low intensity may be because different interlayer spaces were obtained after ionic liquid functionalization, thus suggesting that exfoliation of layered IL-RGO was obtained. FIG. 4 d shows the XRD peaks of TiO₂. The relatively noisy XRD spectrum is likely due to the presence of IL-RGO in the sample. Both anatase and rutile phases are present in the IL-RGO/TiO₂ sample. The anatase TiO₂ accounts for 73% while rutile phase TiO₂ contributes to 27%. The fractional content determined are very similar to the ratio of anatase and rutile of Degussa P25, which is the TiO₂ precursor in this work.

The scanning electron micrographs (SEM) of IL-RGO/TiO₂ and RGO/TiO₂ are shown in FIG. 5. The separated RGO flakes can be clearly seen in the IL-RGO/TiO₂ sample in FIG. 5 a. However, without the functionalized IL as in FIG. 5 b, the RGO particles are much larger in the RGO/TiO₂ sample and aggregate together. A few TiO₂ nanoparticles exist above the RGO aggregates, but the majority of the TiO₂ nanoparticles are covered by the RGO. Due to the thickness of the multi-layer RGO, the TiO₂ nanoparticles below the RGO cannot be clearly seen in the SEM image. When the IL-RGO/TiO₂ solution and the RGO-TiO₂ solution were centrifuged, the IL-RGO and TiO₂ were well mixed while the RGO and TiO₂ were separated. The SEM images reveal that better separation of the graphite layers can be obtained with the IL-RGO material. The presence of the functionalized ionic liquid enhances the solubility of functionalized reduced graphite oxide in water. Thus, a well-mixed IL-RGO and TiO₂ could be obtained in solution.

Photocatalytic Reduction of CO₂ over IL-RGO/TiO₂ and Bare P25 with H₂O Vapor

The experiments on the photoreduction of CO₂ were carried out using a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer equipped with a Praying Mantis diffuse reflectance accessory (Harrick Sci. Corp., Model DRP-M-07) and a 316 stainless steel high temperature reaction chamber (Harrick Sci. Corp., Model HVC-DRP-4), a mercury cadmium telluride (MCT) detector and a KBr beam splitter. The chamber dome has two KBr windows and a quartz window. The quartz window was used for visual observation while the KBr windows were used to permit entry and exit of the infrared beam.

The IL-RGO/TiO₂ or pristine Degussa P25 powders were placed in the sample compartment of the reaction chamber, and the dome was mounted and sealed with an O-ring. The reaction chamber has an inlet for introducing gas to the sample and an outlet for gas exhaust. The chamber was purged with N₂ before introducing the CO₂ and H₂O vapor reactant gases. The H₂O vapor was obtained by flowing N₂ through an impinger containing distilled water. CO₂ mixed with humidified N₂ was introduced to the reaction chamber. Mass flow controllers (Omega Engineering, Inc.) were used to control the flow of CO₂ and N₂. The inlet mixing ratio of CO₂ is 10% by volume. An IR spectrum was obtained after the CO₂/humidified N₂ mixture flowed over the photocatalyst, in order to check that CO₂ and H₂O vapor were absorbed to the surface of the catalyst. The inlet and outlet of the chamber were sealed, and subsequent spectra were taken in a batch mode of operation.

A series of background experiments were conducted in order to characterize the system and to ensure the absence of product formation, even in the presence of the catalytic surface. The sample with CO₂ and humidified N₂ was kept in the dark for 30 minutes. IR spectra were obtained during the 30 minutes dark period to examine whether products formed in the absence of light. In addition, a background experiment with the catalyst and humidified N₂ but without CO₂ was performed under UV-Visible light irradiation in order to determine whether there was product formation in the absence of CO₂. After performing the background experiments, the catalyst (IL-RGO/TiO₂ or P25) was placed in the chamber with CO₂ and humidified N₂. UV-Visible light, produced from a deuterium-halogen light source (Ocean Optics DH-2000-BAL, wavelength=210-1500 nm), was used to activate the catalyst. An optical fiber cable was used to introduce the light to the sample surface through the quartz window of the chamber. Several IR spectra at different irradiation times were acquired over a total irradiation time of 60 minutes. Each spectrum was acquired using 4 cm⁻¹ resolution and 32 scans.

To attempt to quantify the amount of product formed, standard samples of product (diluted by N₂) were admitted to the DRIFTS reaction cell to generate a calibration curve. The gas was allowed to equilibrate with the surface, a spectrum was obtained, and the surface was subsequently purged with N₂ in between each admission of product. Spectra were acquired during the N₂ purge to ensure that the product was completely desorbed from the surface and removed from the chamber. A surface adsorbed product calibration curve was generated and used to attempt to quantify the amount of product formed during the CO₂ photoreduction experiment over IL-RGO/TiO₂.

The CO₂ photoreduction using IL-RGO/TiO₂ catalyst in the presence of H₂O vapor was performed twice using fresh and regenerated catalyst samples. Initially, fresh IL-RGO/TiO₂ was used for CO₂ photoreduction. After 30 minute UV-Visible irradiation, the sample was regenerated by cleaning the catalyst surface using pure N₂. IR spectra of the catalytic surface were obtained to ensure that the reactants (CO₂ and H₂O vapor), and the CH₄ product formed in the previous run were completely dissociated from the catalyst surface and removed from the reaction chamber. Then, the reactants with the same concentrations were admitted to the reaction chamber and the photoreduction experiment was performed again.

In the background experiments, no new peaks were observed in the IR spectra of IL-RGO/TiO₂ with CO₂ and humidified N₂ in the dark over 30 minutes. When IL-RGO/TiO₂ and humidified N₂ (in the absence of CO₂) were irradiated for 30 minutes using UV-Vis light, no new peaks were detected in the IR spectra. Experiments were also performed for bare P25 with CO₂ and humidified N₂. Even after 60 minutes of UV-V is irradiation, no new peak formation was detected in the IR spectra.

The IL-RGO/TiO₂ composite material was applied to the photoreduction of CO₂ in the presence of H₂O vapor. IR spectra of the IL-RGO/TiO₂ surface before and after UV-Visible irradiation were obtained and are shown in FIG. 6. After only 40 seconds of irradiation, new IR features started to appear in the spectrum. In going from 40 seconds to 60 minutes of irradiation, a new peak at 3017 cm⁻¹ continued to grow in intensity. The new peak was initially identified by comparison to the literature as being characteristic of CH₄. In addition, a standard IR spectrum of pure CH₄ over IL-RGO/TiO₂ was created in-house and compared with the product's IR spectrum. A comparison confirmed that CH₄ was indeed the product from reduction of the CO₂ in the presence of water vapor. The background experiments of IL-RGO/TiO₂ with H₂O vapor but without CO₂ showed that no peak formed after 30 minutes of UV-Vis irradiation as evidenced by FIG. 7, thus confirming that CH₄ was indeed formed from CO₂ reduction in the presence of water vapor rather than from other carbon sources (i.e. RGO or the attached ionic liquid).

The major challenge in CO₂ photoreduction is that the recombination of electron and hole generated from TiO₂ is very fast. Thus, there is a significant decrease in the photocatalytic efficiency of TiO₂. P25, which is the mixed phases of TiO₂ with approximately 75% anatase and approximately 25% rutile, is expected to have better electron and hole separation than a single phase of TiO₂ due to the different positions of the conduction and valence bands of the anatase and rutile phases such as is illustrated in FIG. 8.

Electron paramagnetic resonance (EPR) studies by Gray and co-workers indicated that photogenerated electrons actually migrated from rutile to lower energy anatase trapping sites, consequently enhancing electron-hole separation. See, for example: Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C., Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO₂ using EPR. Journal of Physical Chemistry B 2003, 107, 4545-4549; Hurum, D. C.; Gray, K. A.; Rajh, T.; Thurnauer, M. C., Recombination pathways in the Degussa P25 formulation of TiO₂: Surface versus lattice mechanisms. Journal of Physical Chemistry B 2005, 109, 977-980; and Hurum, D. C.; Agrios, A. G.; Crist, S. E.; Gray, K. A.; Rajh, T.; Thurnauer, M. C., Probing reaction mechanisms in mixed phase TiO₂ by EPR. Journal of Electron Spectroscopy and Related Phenomena 2006, 150, 155-163.

However, when P25 was used in the CO₂ photoreduction experiments, no product was observed under the DRIFTS experimental conditions. Therefore, the commercial P25 was still not effective enough for the photoreduction of CO₂ with H₂O vapor. Nevertheless, it was found that the presence of IL-RGO significantly enhances the photoactivity of P25 which is likely due to the improved electron-hole separation via electron transport from the TiO₂ to the IL-RGO.

The production of CO is frequently reported in the literature as the major product for CO₂ photoreduction studies using TiO₂-based nanoparticles. The IR feature of CO is expected to appear in the 2000-2270 cm⁻¹ frequency range. Notably, the lack of CO features in FIG. 6 suggests that there is insignificant production of CO, and CH₄ is the only product of CO₂ photoreduction in the presence of water vapor over IL-RGO/TiO₂.

Additionally, in the literature, there are different proposed mechanisms for CH₄ formation in CO₂ photoreduction using TiO₂-based photocatalysts. The selectivity of the products, for example CO or CH₄, may be due to differences in the numbers of electrons that are produced by the catalyst and separated from the holes. As reported for photochemical reduction of CO₂, the reaction mechanism for CO production requires four electrons, while eight electrons are needed for CH₄ production. The selective formation of CH₄ that is reported herein suggests that more electrons are available from the IL-RGO/TiO₂ material as compared to the catalysts used in published studies that have CO as the major product of CO₂ photoreduction. This further confirms that the presence of IL-RGO helps to separate electron and hole pairs. The selective production of CH₄ is very valuable in the application of the catalyst in CO₂ photoreduction because CO, as a synthesis gas, cannot be used as a fuel directly whereas CH₄ is an energy-rich fuel.

In FIG. 10, the concentration of CH₄ formed at different UV-Visible irradiation times by fresh IL-RGO/TiO₂ catalyst and regenerated IL-RGO/TiO₂ catalyst are shown. The results show that the regenerated sample can produce an almost identical amount of CH₄ at the same irradiation times. These preliminary results suggest that the IL-RGO/TiO₂ catalyst can be regenerated and effectively reused in CO₂ photoreduction. This result is significant in that it signifies the real-world application of this material in CO₂ photoreduction. The CH₄ production rate of 279 μmol/g catalyst-hr over a 55 minute period was calculated. The CH₄ production rate is about 20-30 times higher as compared to the highest CH₄ production rate using other modified TiO₂ structure reported in the literature.

Although the present invention has been described in detail with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the invention should not be limited to the description of the embodiments contained herein. For example, it is contemplated that another application for this material or other materials made by similar processes may exist in Field Effect Transistors. 

What is claimed is:
 1. A photocatalytic nanocomposite comprising: a reduced graphite oxide; a photocatalytic metal oxide in nanoparticle form; wherein the photocatalytic metal oxide in nanoparticle form is dispersed in the reduced graphite oxide.
 2. The photocatalytic nanocomposite of claim 1, further comprising an ionic moiety attached to the reduced graphite oxide.
 3. The photocatalytic nanocomposite of claim 2, wherein the ionic moiety comprises R₁R₂R₃ and wherein R₁ is NH, R₂ is alkylene, and R₃ is a cationic group.
 4. The photocatalytic nanocomposite of claim 3, wherein R₂ is C₁ to C₅ alkylene, and R₃ is an imidazole ring protonated or substituted at a nitrogen atom.
 5. The photocatalytic nanocomposite of claim 4, wherein R₂ is propylene, and R₃ is an alkyl-substituted imidazole ring.
 6. The photocatalytic nanocomposite of claim 1, wherein the reduced graphite oxide is an ionic liquid functionalized reduced graphite oxide formed by attaching a NH₂-terminated ionic liquid to the reduced graphite oxide.
 7. The photocatalytic nanocomposite of claim 1, wherein the photocatalytic metal oxide is TiO₂.
 8. The photocatalytic nanocomposite of claim 7, wherein the TiO₂ is in the form of rutile and anatase.
 9. The photocatalytic nanocomposite of claim 1, wherein the reduced graphite oxide is a powder.
 10. A method of making a photocatalytic nanocomposite comprising: oxidizing graphite to form a reduced graphite oxide; mixing the reduced graphite oxide with TiO₂ nanoparticles to form the photocatalytic nanocomposite.
 11. The method of claim 10, further comprising the step of functionalizing the reduced graphite oxide with a NH₂-terminated ionic liquid to form an ionic liquid functionalized reduced graphite oxide before the step of mixing.
 12. The method of claim 11, wherein the NH₂-terminated ionic liquid is an imidazole.
 13. The method of claim 11, wherein the NH₂-terminated ionic liquid is a 1-butyl-3-methylimidazolium-based ionic liquid.
 14. The method of claim 11, wherein the NH₂-terminated ionic liquid is a 1-butyl-3-methylimidazolium chloride.
 15. A method of CO₂ photoreduction comprising: contacting reactants of CO₂ and H₂O over a photocatalytic nanocomposite according to claim 1; reacting the CO₂ and H₂O over the photocatalytic nanocomposite to produce products including CH₄.
 16. The method of claim 15, wherein the products are substantially free of CO gas.
 17. The method of claim 15, wherein the CH₄ has a production rate in excess of 10 μmol/g catalyst-hr.
 18. The method of claim 15, wherein the CH₄ has a production rate in excess of 250 μmol/g catalyst-hr. 